The invention relates to aqueous crosslinkable binder dispersions comprising hydroxy- and acid-functional copolymers based on ethylene glycol monoallyl ether, to a process for preparing them and to the use of the dispersions in coating materials.
It is known and is widespread practice in the coatings industry to use binders based on hydroxy-functional copolymers in coating systems. These binders can be used in organically dissolved form, as an aqueous dispersion, and in water-dilutable form in solventborne or aqueous coating materials. It is possible to crosslink these copolymers in the coating material via the OH groups, using for example melamine resins or (blocked) polyisocyanates, to form coating materials which can be processed as one-component or two-component systems.
Aqueous coating systems are increasingly being used in order to lower the emissions of volatile organic compounds (VOCs). As compared with the solvent-based coating materials, they contain a significantly reduced fraction of organic solvents (“cosolvents”). Often, however, the changeover to aqueous coating systems can result in deterioration in other coating properties, for example the solvent resistance and chemical resistance. The reason for this is the higher hydrophilicity of the aqueous or water-dilutable binders as compared with their solvent-based counterpart.
One requirement which is particularly difficult to fulfil is that of anti-graffiti resistance, i.e., the paint sprays, color sticks or paint sticks which are, for example, commercially customary leave behind spots of color in the paint film which cannot be removed using cleaning products without damaging the paint film (for example, impairing the gloss) (the requirements are described, for example, in TL 918 300, section 4.11, Deutsche Bahn AG). The resistance of aqueous coatings based on hydroxy-functional copolymer dispersions can be improved by combining them in the coating material with polyisocyanates containing free NCO groups, as crosslinkers, to form aqueous two-component polyurethane (2K-PU) coating materials. Such coating materials are described, for example, in EP-A 0 358 979 or in EP-A 0 947 557. The copolymers on which these coating dispersions are based are prepared by free-radical polymerization of vinyl monomers in aqueous phase, referred to as primary dispersions, or in organic phase with subsequent dispersion in water, referred to as secondary dispersions. The monomer mixtures generally include hydroxy (meth)acrylate monomers as building blocks for the OH functionality of the resin. This produces coating films having a high level of coatings properties overall, and yet the anti-graffiti resistance of these coating films does not meet the requirements.
U.S. Pat. No. 5,475,073 discloses hydroxy-functional copolymer resins in which the OH groups have been introduced by incorporating allyl alcohol or propoxylated allyl alcohol. EP-A 0 900 243 and WO-A 00/00527 describe water-dilutable, hydroxy-functional resins for coating compositions, the OH groups having been introduced into the resins by incorporating propoxylated allyl alcohol.
WO-A 99/51658 discloses two-component polyurethane coating materials comprising aqueous dispersions of polyisocyanates containing free NCO groups and hydroxy-functional resins, based on allyl alcohol or on an alkoxylated allyl alcohol.
An object of the present invention was to provide binder dispersions based on hydroxy-functional resins with which it is possible to prepare aqueous 2K-PU coating materials having good film optical properties and good resistance properties, in particular very good anti-graffiti resistance.